1. Field of the Invention
This invention relates to a projection television system including three cathode ray tubes assembled for the respective three primary colors, i.e., red, green, and blue, having a multilayered interference filter being composed of a number of layers and disposed on the inside of a display window.
2. Description of the Related Art
FIG. 3 of the accompanying drawings shows an existing projection television system. In FIG. 3, a reference numeral 1 designates a screen; 2, a blue cathode ray tube (hereinafter abbreviated as CRT) comprising a display window 2A and a multilayered interference filter 12 disposed on the inside of the display window, the filter 12 being composed of a number of layers which are alternately manufactured from a high refractive index material and a low refractive index material; and 3, a green CRT comprising a display window 3A and a multilayered interference filter 12 disposed on the inside of the display window, the filter being composed of a number of layers which are alternately manufactured from a high refractive index material and a low refractive index material.
Further, 5 designates a system of blue projection lenses which project an image light emitted from the blue CRT 2 onto the screen 1; and 6, a system of green projection lenses which project an image light emitted from the green CRT 3 onto the screen 1.
Furthermore, 10 designates a red CRT which comprises a display window 10A and, like the blue and green CRTs, also a multilayered interference filter 12 disposed on the inside of the display window, the filter being composed of a number of layers manufactured in the manner described above. A system of red projection lenses 7 are located forwardly of the red CRT and project an image light onto the screen.
FIG. 4 is a magnified diagram of an area encircled with dot lines 11 in FIG. 3. In FIG. 4, reference numeral 12 designates a multilayered interference filter; and 9, a layer of fluorescent material formed on the innermost surface of the display window. The multilayered interference filter 12 is disposed between the display window and the fluorescent material layer.
In operation, transmittivity characteristics of each multilayered interference filter 12, which is disposed on the inside of the three display windows 2A, 3A, 10A of the blue, green and red CRTs 2, 3 and 10, is adjusted to be close to a desired central wavelength selected from the emission spectrum of fluorescent material layers 9 of each color. The luminescent intensity distribution is improved by 50% to 60% within the aperture angle ranging from zero to about thirty-five degrees as shown by solid lines 14 in FIG. 4 as compared with a tube without an interference filter, as shown by dot lines 13.
An existing projection television system produces an image onto the screen 1 with high brightness by projecting the lights emitted from the blue, green, red CRTs 2, 3, 10 having the multilayered interference filter 12 via the projection lens of each color.
A problem with the foregoing conventional arrangement is that the production cost of television systems are of increased proportion as the yield impaired by multilayered interference films obliquely manufactured and in consideration of evaporation time fluctuation because a multilayered interference filter disposed on the inside of the display window of each CRT has a structure manufactured from about eleven to twenty layers of evaporation films.